White Light Emitting Structures with Controllable Emission Color Temperature

ABSTRACT

Disclosed herein are multi-layered optically active regions for semiconductor light-emitting devices (LEDs) that incorporate intermediate carrier blocking layers, the intermediate carrier blocking layers having design parameters for compositions and doping levels selected to provide efficient control over the carrier injection distribution across the active regions to achieve desired device injection characteristics. Examples of embodiments discussed herein include, among others: a multiple-quantum-well variable-color LED operating in visible optical range with full coverage of RGB gamut, a multiple-quantum-well variable-color LED operating in visible optical range with an extended color gamut beyond standard RGB gamut, a multiple-quantum-well light-white emitting LED with variable color temperature, and a multiple-quantum-well LED with uniformly populated active layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 15/173,511, filed Jun. 3, 2016, which claims the benefit ofU.S. Provisional Patent Application No. 62/171,536, filed Jun. 5, 2015,U.S. Provisional Patent Application No. 62/275,650, filed Jan. 6, 2016,and U.S. Provisional Patent Application No. 62/290,607, filed Feb. 3,2016, the contents of each of which are hereby incorporated by referenceas if fully stated herein.

FIELD OF THE INVENTION

The disclosure herein relates to means for tailoring and controlling thedistribution of charge carriers in multi-layer active regions ofelectrically pumped solid state light emitters such as light-emittingdiodes and laser diodes.

BACKGROUND OF THE INVENTION

Semiconductor light emitting diode structures have become theestablished leaders among optical light sources in different emissionwavelength ranges. A conventional light-emitting structure is typicallycomprised of multiple layers of III-V compound semiconductor alloys suchas GaAsP, AlGaAs, AlGaInP, or AlGaInN, depending upon the targetedwavelength emission of the diode structure. III-nitride AlGaInN alloystake special place among the possible material systems because of thewide range of available band-gaps. Light emission from AlGaInN coversthe entire visible spectrum; III-nitride based light sources arecurrently being developed also for ultraviolet and infrared emission.Multiple active layer design of an optoelectronic device active regionoffsets a high level of optical and electrical losses and small strainrelaxation length typical for III-nitride based heterostructures.

Multiple quantum well (MQW) design of the optically active region isbeneficial for light emitter performance. By increasing the number ofactive quantum wells (QWs), the injected carriers can be spread amongthe MQWs thus decreasing the average QW population and minimizing theadverse effects of (i) nonradiative Auger recombination, (ii) QW thermaldepopulation, and (iii) QW optical transition saturation. On the otherhand, MQW active regions of electrically pumped devices typically sufferfrom inhomogeneous distribution of charge carriers, both electrons andholes, which are injected from the opposite sides of the diodestructure. As a result, uneven and imbalanced population of active QWsunfavorably affects the device performance. In III-nitridelight-emitting diodes (LEDs), the overpopulated active QWs oftenintensify the device efficiency droop either by increasing thenonradiative Auger recombination loss or by elevating the carrierleakage from the device active region. In laser diodes (LDs), theunder-pumped QWs append their inter-band absorption to the total opticalloss thus increasing the laser threshold.

In polar III-nitride heterostructures, non-uniform carrier injection isadditionally aggravated by built-in polarization fields and relatedpotential barriers. This sometimes makes nonpolar or semipolartechnology an attractive alternative to polar templates. Nonpolartemplates, however, do not solve the problem of inhomogeneous injectionentirely. Even in the absence of internal polarization fields, MQWstructures with sufficiently deep QWs and strong carrier confinementreveal uneven QW populations in a wide range of injection currents, sothat the carrier population non-uniformity in III-nitride MQWs is acommon feature of both polar and non-polar templates. Carrier injectioninhomogeneity increases with active QW depth and, therefore, becomesmore pronounced in the longer-wavelength emitters thus holding back theefficiency of III-nitride based light emitters in the so-called “greenemission gap”.

Several conventional methods employ an MQW active region design inattempt to achieve multi-color emission with fixed or variable emissioncolors and/or to increase the injection efficiency of the device activeregion. For example, U.S. Pat. No. 7,323,721 describes a monolithicmulti-color MQW structure designed to emit white light by including asufficient number of QWs with different emission wavelengths, while U.S.Pat. No. 8,314,429 describes a multi junction light emitting structurewith the MQWs of each junction being designed to emit a specificwavelength that combines into white-light emission depending on thedesigned emission intensity of each of the multiple junctions comprisingthe structure. U.S. Pat. Nos. 7,058,105 and 6,434,178 describeapproaches to achieve high carrier injection efficiency by incorporatingmeans for increased optical and, respectively, electrical confinementsof MQW active region. U.S. Patent Publication No. 2011/0188528 describesa MQW III-nitride light-emitting diode structure that achieves highcarrier injection efficiency by using shallow QWs designed to avoidexcessive carrier confinement and attain uniform MQW carrier population.U.S. Patent Publication No. 2010/0066921 describes a MQW III-nitridelight emitting structure epitaxially grown on micro rods in which theepitaxial growth plane of the micro rods promotes higher indiumincorporation in the semi-polar and non-polar orientation which can leadto multi-color emission form the MQW structure. Thus, the foregoingconventional approaches use particular ad hoc approaches relevant totheir specific goals.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein are illustrated by way of example and not by wayof limitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment of the disclosure are not necessarily to thesame embodiment, and they mean at least one. Also, in the interest ofconciseness and reducing the total number of figures, a given figure maybe used to illustrate the features of more than one embodiment of thedisclosure, and not all elements in the figure may be required for agiven embodiment.

FIG. 1 illustrates a typical energy-band profile of an active region forexplaining a conventional hetero junction multi-layerquantum-confinement-based light-emitting diode structure.

FIG. 2 illustrates an active region energy-band profile for explainingthe layout of a hetero-junction multi-layer quantum-confinement-basedlight-emitting diode structure according to an embodiment disclosedherein.

FIG. 3 illustrates a simplified schematic cross-sectional view forexplaining a hetero-junction multi-layer quantum-confinement-basedlight-emitting diode structure according to an embodiment disclosedherein.

FIGS. 4A-4D illustrate results of active region band profile simulationsin three-color Red-Green-Blue (RGB) LED structures according to exampleembodiments disclosed herein. In FIGS. 4A and 4B, the three-colorRed-Green-Blue (RGB) LED structure is designed according to theconventional arrangement of FIG. 1 without intermediate carrier blockinglayers (IBLs). In FIGS. 4C and 4D, the three-color Red-Green-Blue (RGB)LED structure is designed according to an embodiment disclosed hereinusing IBLs. In FIGS. 4A-4D, band profiles are calculated at an LEDinjection level of 10 A/cm² and the band gap vertical scale is reducedby 1 eV for illustrative purposes. FIG. 4C illustrates the active regionband profile for the RGB-IBL-LED structure of FIG. 3 with IBLs designedfor the purpose of emission color control.

FIGS. 5A-5D illustrates a comparison of CIE chromaticity diagramscalculated in a range of injection currents for three-color RGB LEDstructures explained using FIGS. 4A-4D according to example embodimentsdisclosed herein. The structures for FIGS. 5A-5D correspond to thestructures for FIGS. 4A-4D, respectively. In FIGS. 5A-5D, a circlemarker indicates starting point of low injection current, a squaremarker indicates the end point of high injection current, and a trianglemarker corresponds to nominal injection level of 10 A/cm² used in FIGS.4A-4D. The various crosses (“X”) indicate standard RGB color gamutprimaries of different colors. FIG. 5C illustrates the emission colorcontrol achieved in the RGB-IBL-LED structure illustrated by FIG. 3 andFIG. 4C.

FIGS. 6A-6B illustrate the details of a color control process in theRGB-IBL LED structure illustrated by FIG. 3 and FIG. 4C according toexample embodiments disclosed herein. FIG. 6A illustrates the injectiondependence of the CIE chromaticity coordinates for controlled RGBemission. FIG. 6B illustrates the injection dependence of relativeoptical emission power from each QW. For comparison, dashed lines inFIGS. 6A-6B indicate corresponding dependencies for the RGB(3)-IBL LEDillustrated by FIG. 4D.

FIGS. 7A-7F illustrate sensitivity of MQW carrier injection and outputcolor control to the design of IBL elements of the LED structureaccording to example embodiments disclosed herein. Each of FIGS. 7A-7Fillustrates the CIE chromaticity diagram of an RGB-IBL LED similar to300 of FIG. 3 with compositions of the two intermediate carrier blockinglayers (IBL1 and IBL2) having modified design parameters.

FIGS. 8A-8D illustrate processes of designing RGB-IBL LED structuresaccording to example embodiments disclosed herein. Each of FIGS. 8A-8Dillustrates the CIE chromaticity diagram of an RGB-IBL LED such asstructure 300 FIG. 3 with designed IBL1 and IBL2 compositions and varieddoping of the first intermediate carrier blocking layers (IBL1).

FIGS. 9A-9D illustrate the injection dependence of the CIE chromaticitycoordinates for the RGB-IBL LED structures of FIGS. 8A-8D. FIGS. 9A-9Dillustrate CIE chromaticity coordinates for an RGB-IBL LED such asstructure 300 of FIG. 3 with designed IBL1 and IBL2 compositions andvaried doping of IBL1.

FIGS. 10A-10D illustrate a process of designing an RGB-IBL LED structureaccording to example embodiments disclosed herein. FIGS. 10A-10Dillustrate CIE diagrams for an RGB-IBL LED such as structure 300 of FIG.3 with designed IBL1 and IBL2 compositions and varied p-doping level inIBL2.

FIG. 11A illustrates an embodiment in which color emission is extendedbeyond the standard RGB palette by including an additionalaquamarine-emitting QW and using intermediate blocking layers.

FIG. 11B-11C relate to an embodiment in which an additional intermediatecarrier blocking layer is incorporated to reduce the full color controlcurrent injection range. FIG. 11B illustrates the band profile of anRGB-IBL LED structure in which an additional intermediate carrierblocking layer is incorporated to reduce the full color control currentinjection range. FIG. 11C illustrates the injection dependence of thelight emission gamut CIE RGB chromaticity coordinates of the RGB-IBL LEDstructure in which an additional intermediate carrier blocking layer isincorporated.

FIG. 12A illustrates a comparison of a calculated variable-coloremission spectra with experimental electroluminescence spectra obtainedfrom a monolithic color-tunable nitride-based three-color RGB-IBL LEDepitaxially grown according to example embodiments disclosed herein.

FIG. 12B is a view for explaining output emission colors at differentinjection currents obtained from a monolithic color-tunablenitride-based three-color RGB-IBL LED epitaxially grown according toexample embodiments disclosed herein.

FIG. 12C illustrates emission colors gamut coverage at differentinjection currents obtained from a monolithic color-tunablenitride-based three-color RGB-IBL LED epitaxially grown according toexample embodiments disclosed herein.

FIG. 13 illustrates experimental electroluminescence spectra of anmonolithic color-tunable nitride-based three-color RGB-IBL LED such asstructure 300 of FIG. 3 grown according to example embodiments disclosedherein and illustrates a detailed comparison with simulation results.

FIG. 14 illustrates a CIE diagram and injection dependence of emissionchromaticity coordinates calculated for an RGB-IBL LED such as structure300 of FIG. 3 designed to produce white-light emission at maximum LEDefficiency point according to example embodiments disclosed herein.

FIG. 15 illustrates the experimental electroluminescence spectra of amonolithic broadband visible light-emitting nitride-based RGB-IBL LEDsuch as structure 300 of FIG. 3 designed and grown according to exampleembodiments disclosed herein.

FIG. 16 illustrates a comparison of calculated emission characteristicsof monochromatic MQW LEDs designed according to a conventionalarrangement with a MQW-IBL LED designed to sustain uniformly populatedactive layers at maximum LED efficiency point according to exampleembodiments disclosed herein.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments and aspects of the disclosure herein will bedescribed with reference to details discussed below, and theaccompanying figures will illustrate the various embodiments. Thefollowing description and figures are illustrative of the disclosureherein and are not to be construed as limiting. Numerous specificdetails are described to provide a thorough understanding of variousembodiments. However, it is understood that embodiments disclosed hereinmay be practiced without these specific details. In certain instances,well-known or conventional details, such as circuits, structures, andtechniques, are not described in order to provide a concise discussionof example embodiments.

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin conjunction with the embodiment can be included in at least oneembodiment of the invention. The appearances of the phrase “in oneembodiment” in various places in the specification do not necessarilyall refer to the same embodiment.

The inventors herein have recognized the desirability to provide asystematic approach for tailoring a charge carrier populationdistribution and controlling an active layer population uniformity inmulti-layered active regions of semiconductor light-emitting structures,such that it becomes possible to have uniform carrier populationdistribution in the active layer and emitter efficiency is enhanced.Also, the inventors herein have recognized that it is desirable todeliberately tailor or directly control the carrier populationdistribution among the active layers of the light-emitting structure,such that it becomes possible to create monolithic multi-colorsemiconductor light emitters with fixed pre-designed emission spectrum(for instance a white-color emitter) and to develop light emitters withvariable emission color, among many possible other applications. As willbe illustrated by the following descriptions and figures, a systematicmethod is disclosed herein for achieving selective and controllablecharge carrier injection into multiple active layers of solid statelight-emitting diode structures. For example, according to an embodimentherein, multi-layered solid state light emitter structures are designedand epitaxially grown incorporating means for tailoring the chargecarrier transport in the device multi-layer active region andcontrolling the active layer carrier populations for the numerousapplications that benefit from such capability. Also disclosed hereinare examples of numerous possible applications, for example, displayapplications and general lighting applications, high-efficiency solidstate light emitters, multi-color monolithic semiconductor light sourceswith fixed or variable emission wavelength, and white-lightsemiconductor emitters.

According to one aspect of the disclosure herein, a systematic methodfor tailoring and controlling the charge carrier population distributionin multi-layered active regions of semiconductor light emitterstructures such as light emitting diodes and laser diodes is provided bymeans of incorporating into the device active region intermediatecarrier blocking layers (IBLs) that are specifically designed incomposition and doping.

According another aspect of the disclosure herein, targeted modificationof the injection characteristics of multi-layered light-emittingstructures is provided by (i) balancing the asymmetry in electron andhole transport between the optically active layers and (ii) balancingthe carrier capture rates into the optically active layers, thusincreasing the overall active region injection efficiency and reducingthe active region overflow, carrier leakage, and carrier recombinationloss outside the active region.

According yet another aspect of the disclosure herein, a monolithicsemiconductor light-emitting device is provided with controllablesequential carrier injection in a multi-layered active region.

According to an embodiment disclosed herein, a monolithic semiconductorlight-emitting device is provided with uniformly populated active layersin a multi-layered active region.

According to another embodiment disclosed herein, a monolithicsemiconductor light-emitting device is provided with controllable andtunable multi-color emission with fixed or variable emission wavelength.

According to yet another embodiment disclosed herein, a monolithicsemiconductor light-emitting device is provided with controllable andtunable multi-color emission that matches a specified color emissiongamut.

According to still another embodiment disclosed herein, a monolithicsemiconductor light-emitting device is provided with controllable andtunable multi-color emission with wide a color emission gamut.

According to one embodiment disclosed herein, a monolithic semiconductorwhite-light-emitting device is provided with controllable emission colortemperature.

According to an embodiment disclosed herein, a monolithic semiconductorlight-emitting device is provided with single color emission thatattains high internal quantum efficiency (EQE).

In the following descriptions, III-nitride semiconductor alloys are usedas an example material system, since III-nitride semiconductor alloysare poised to make a significant impact in light emission applications.In addition, multiple-quantum-well (MQW) active regions are used in thisdisclosure as an example active region design, since quantum well (QW)heterostructures are commonly employed as quantum confinement structuresfor optically active layers. It will be understood that the embodimentsdisclosed herein are also applicable to other materials systems andoptically active layers incorporating other means of quantum confinementsuch as the use of quantum wires and quantum dots.

Turning to FIG. 1, FIG. 1 illustrates a typical band profile of anactive region of a conventional hetero-junction based light-emittingdiode structure. Light-emitting diode structure 100 is comprised of ann-doped layer 110, an optically active region 130, and a p-doped layer120. Optically active region 130 of the diode structure 100 is oftencompleted with the electron blocking layer (EBL) 140 placed outside ofthe optically active region 130 at the p-side of the diode structure100. In quantum-confinement based LEDs, multi-layered optically activeregion 130 can be further comprised of the multiple quantum well (MQW)layers 131 separated by the quantum barrier layers 132. The III-Vmaterial alloy composition in the optically active MQW layers 131 ofactive region 130 would typically be selected to set up the desiredactive layer band gap and, consequently, the fixed emission wavelengthof the optically active region 130 of the light-emitting diode structure100. Electron blocking layer (EBL) 140, which is incorporated in thediode structure 100 to reduce the electron leakage at the structurep-side, would typically be a heavily p-doped wide-band-gap layer havinga band gap that is greater than the band gap of the barrier layers 132of the active region 130 of the diode structure 100. Averting carrierleakage has been a valuable mechanism to increasing light emitterefficiency, and EBL structures of different kinds are now often standardfeatures of most III-nitride LED designs. It should be emphasizedhowever that carrier blocking layers located outside of the opticallyactive region have little or no effect on the uniformity or carrierdistribution inside the active region and therefore cannot typically beused as a design element to control or tailor the active layer carrierpopulations in multi-layer light-emitting devices. In fact, such asituation might unfavorably affect the carrier distribution inside theactive region due to possibly causing an excessive polarization field,especially in active regions of long wavelength emitters.

FIG. 2 illustrates an active region band profile for explaining ahetero-junction multi-layer quantum-confinement-based light-emittingdiode structure according to one embodiment. Light-emitting diodestructure 200 is comprised of an n-doped layer 210, an optically activeregion 230, and a p-doped layer 220. In the embodiment of FIG. 2,optically active region 230 of the diode structure 200 includes electronblocking layer (EBL) 240 placed outside of the optically active region230 at the p-side of the diode structure 200. In other embodiments, EBL240 is not included. Multi-layered optically active region 230 can befurther comprised of the multiple quantum well (MQW) layers 231separated by the quantum barrier layers 232. In the embodiment of FIG.2, specially designed intermediate carrier blocking layers (IBLs) 233with selected band-gaps, as defined by their respective compositions,and band offsets, as affected by their doping levels, are incorporateddirectly into the optically active region 230 to provide means forcontrolling the carrier transport between the active layers 231 andbalancing the carrier capture rates by active layers 231. The multiplelayers of the light-emitting diode structure of FIG. 2 can be composedof III-V compound semiconductor alloys such as GaAsP, AlGaAs, AlGaInP,or AlGaInN, depending upon the targeted wavelength emission of the diodestructure. A diode structure according to the embodiment of FIG. 2 isapplicable to solid state light emitting diode structures with eitherpolar, or semi-polar, or non-polar crystalline structures havingoptically active layers incorporating means of quantum confinement suchas quantum wells, quantum wires or quantum dots.

FIG. 3 illustrates a simplified schematic cross-sectional view forexplaining a monolithic three-color hetero-junction multi-layerquantum-confinement GaN-based LED device structure 300 according oneembodiment. In the embodiment of FIG. 3, LED structure 300 is formed bymetal organic chemical vapor deposition (MOCVD). However, LED structure300 may be formed by any epitaxial deposition technique. LED structure300 comprises a suitable substrate 311 for epitaxial deposition thereon(e.g., a GaN, Si, sapphire (Al₂O₃), or silicon carbide (SiC) wafer witha nucleation/buffer layer 312 formed thereon) and an overlying N-dopedlayer 310 of the active region 330 (e.g., comprising Si as N-typedopant.) Overlying the N-layer 310 is multi-layered optically activeregion 330 (e.g., comprised of combination of GaN, InGaN, AlGaN, andAlInGaN layers.) Overlying the active region 330 is an optional electronblocking layer (EBL) 340 typically comprised of AlGaN and followed byP-doped layer 320 (e.g., comprising Mg as P-type dopant.) Finally,electrodes 313 and 314 are provided for electrically contacting N- andP-doped layers 310 and 320, respectively.

Still referring to FIG. 3, multi-layer (MQW) active region 330 comprisesa stacked plurality of quantum barrier layers 332 with respectivequantum well optically active layers 331, for blue- green- and red-lightemission. Each of the barrier layers 332 is about 5-20 nm-thick andcomprised mainly of GaN; optically active layers 331 are each about 2-3nm-thick and comprised of InGaN. As shown in FIG. 3, the opticallyactive layers 331 may comprise a red-emitting layer, a green-emittinglayer and a blue-emitting layer. It will be appreciated that in otherembodiments, other optically active layers may be used, includingadditional red-green- and blue-emitting layers, as well asaquamarine-emitting layers.

The multi-layer active region 330 also comprises specially designedintermediate carrier blocking layers (IBLs) 333 which, in thisembodiment, include an additional amount of aluminum (Al) and doping toadjust their respective band-gap and band offsets. Selection of designparameters for intermediate blocking layers (IBLs) 333 functions toselectively control the population of injected carriers inside therespective quantum wells in order to enhance their spontaneous lightemission at a given carrier injection level. In the embodiment of FIG.3, the band-gap of an intermediate carrier blocking layer (IBL) isgreater than that of the associated quantum well layer and its thicknessis selected to avoid high forward voltage and excessive heating whichcan lead to performance degradation. However, other band-gaps andthicknesses may be selected.

As will be explained in further detail below, optically active layers331 in the structure can each be comprised of a multiplicity of QWlayers separated by additional barrier layers 332 as required to achievefull coverage of the device emission color gamut. In this regard, someof the following descriptions describe modifications to the structureillustrated in FIG. 3, such that FIG. 3 is merely one example of thearrangement and number of quantum barrier layers, active layers andintermediate blocking layers included in the LED structure.

Moreover, multiple-quantum-well (MQW) active regions are used as quantumconfinement structures in the embodiment of FIG. 3. However, otherquantum confinement structures may be used, such as a quantum well orwells, quantum wires and quantum dots. In this regard, in embodimentsinvolving quantum wires and quantum dots, the combination of a quantumbarrier layer, an active layer and the quantum barrier layer on theother side of the active layer as shown in FIG. 3 is replaced by quantumwires or quantum dots as the quantum confinement structures.

FIGS. 4A and 4B illustrate results of an active region band profilesimulation for multiple-color Red-Green-Blue (RGB) LED structuresdesigned according to the embodiment of FIG. 1 (FIG. 4A relates to anundoped RGB structure; FIG. 4B relates to a doped RGB structure). FIGS.4C AND 4D illustrate results of an active region band profile simulationfor multiple-color Red-Green-Blue (RGB) LED structures designedaccording to the embodiment of FIG. 2 in which IBLs 233 have beenincorporated into the active region (FIG. 4C relates to an RGB-IBLstructure; FIG. 4D relates to an RGB(3)-IBL structure). The simulationresults shown in FIGS. 4A-4D were obtained by using a simulationprogram, such as Optical Device Modeling & Simulation (ODMS) software.For clarity of results, the EBL layer has been excluded from thesesimulations.

FIGS. 4A and 4C illustrate the active region band profiles of thestructures of FIG. 1 and FIG. 2, respectively. FIG. 4C illustrates thesimulated active region energy band profiles of an RGB-IBL LED devicestructure with the active layer band-gaps and the IBLs compositions anddoping levels selected for variable (or tunable) color control over adesired light emission gamut. All LED energy band profiles illustratedin FIGS. 4A-4D are simulated at approximately the same LED injectionlevel of 10 A/cm². The dashed lines in FIGS. 4A-4D show the quasi-Fermilevels for electrons (lines 401, 404, 407, 410) and holes (lines 403,406, 409, 412). The dashed-dotted lines (lines 402, 405, 408, 411) inFIGS. 4A-4D indicate the internal potential distribution inside theactive region.

Table 1 provides examples of active region layout design parameters ofthe designed RGB-IBL LED structure of FIG. 4C, including thesemiconductors material composition of each of the structure's layersand their respective doping levels. Actual emission results of LEDstructures epitaxially grown in accordance with the disclosure hereinare discussed below and confirm the described simulation results for anRGB-IBL LED structure for FIG. 4C.

TABLE 1 Donors Acceptors Width (10¹⁷ (10¹⁷ Layer nm Composition cm⁻³)cm⁻³) P-Layer 250 GaN 0.0 100.0 EBL 15.0 Al_(0.20)Ga_(0.80)N 0.0 500Barrier 10.0 GaN 0.0 5.0 Red QWs 3.0 Ga_(0.65)In_(0.35)N 0.0 0.0 Barrier5.0 GaN 0.0 0.0 IBL 2 (Green-Red) 10.0 Al_(0.20)Ga_(0.80)N 0.0 15.0Barrier 5.0 GaN 0.0 0.0 Green QWs 3.0 Ga_(0.75)In_(0.25)N 0.0 0.0Barrier 5.0 GaN 0.0 0.0 IBL1 (Blue-Green) 10.0 Al_(0.07)Ga_(0.93)N 0.00.0 Barrier 5.0 GaN 0.0 0.0 Blue QWs 3.0 Ga_(0.85)In_(0.15)N 0.0 0.0Barrier 10.0 GaN 1.0 0.0 N-Layer 500 GaN 10.0 0.0

In another embodiment, which is illustrated by FIG. 4D, additionalactive layers are incorporated into the diode structure active region inorder to balance the device light emission gamut. According to thisembodiment, the RGB-IBL device active region incorporates additionalBlue-emitting QWs introduced to improve RGB light emission gamutcoverage. Adding more Blue-emitting QWs increases the total carriercapture by the Blue-emitting QWs, thus extending the emissioncharacteristics of the RGB-IBL LED device to include a desired coloremission primary (blue, in the embodiment of FIG. 4D) of the lightemission gamut.

FIGS. 5A-5D compare light emission gamut (CIE chromaticity diagrams) forthe multi-color LED structures of FIGS. 4A-4D over a range of injectioncurrents. The embodiments of FIGS. 5A and 5C correspond to theembodiments of FIGS. 4A and 4C, respectively. In the FIGS. 5A-5D, thecircle markers indicate the starting point at the lowest injection(simulated value of 1 μA/cm²), the square markers correspond to thehighest injection level (simulated value of 10 kA/cm²), the trianglesmark the injection level (simulated value of 10 A/cm²) used in FIGS.4A-4D, and the crosses (“X”) indicate the primary colors of a standardRGB gamut (red is indicated by crosses 502, 506, 510, 514; green isindicated by crosses 501, 505, 509, 513; blue is indicated by crosses503, 507, 511, 515), such as an HD color gamut, for example. FIGS. 5Cand 5D illustrate embodiments in which RGB-IBL devices are designed withselected IBLs band-gaps and band-offsets to provide a controllablecarrier injection distribution within a device active region and tunablemulti-color emission covering the standard RGB gamut. FIG. 5D, inparticular, shows an embodiment in which the emission of the RGB-IBL LED300 of FIG. 3 (and also of FIG. 4C) is further improved to achieve fullcoverage of the standard RGB gamut by adding two Blue-emitting QWsbefore growth of the associated barrier layer and the first IBL (hereinreferred to as, IBL1).

FIGS. 6A-B detail the process of color control in the RGB-IBL LED 300 ofFIG. 3 (and also of subplot C of FIG. 4). In FIGS. 6A-B, lines 606 and611 indicate red; lines 604 and 609 indicate green, lines 602 and 608indicate blue. FIG. 6A illustrates the injection dependence of the lightemission gamut CIE RGB chromaticity coordinates. FIG. 6B illustrates therelative optical emission power in each QW color group, P_(i)/ΣP_(i)where index i relates to a Red-, Green, or Blue-emitting QW,respectively. Dashed lines (lines 601 and 607 indicate blue; lines 603and 610 indicating green; lines 605 and 612 indicating red) show thecorresponding injection dependencies for RGB-IBL(3) light emittingstructure incorporating three Blue-emitting QWs illustrated in plot 470of FIG. 4. It should be noted that additional optically active QW layers331 separated by additional barrier layers 332 are added in the devicestructure of FIG. 3 as required to achieve full coverage of the deviceemission color gamut and/or a desired P_(i)/Σ_(i) relative opticalemission power in each QWs color group. Accordingly, the structure ofFIG. 3 may include fewer or additional optically active QW layers 331and, correspondingly, fewer or additional barrier layers 332 to separatethem.

Referring again to FIGS. 5A-5D, a comparison of the light emission gamutof FIGS. 5A and 5C (which also correspond to FIGS. 4A and 4C,respectively) illustrates the effect of incorporating the IBLs withselected band-gaps and band-offsets into the active region of asemiconductor light emitting diode structure that incorporates multipleactive layers with band-gaps corresponding to a multi-color lightemission. As can be seen from FIG. 5A, without the IBLs, the coloremission trajectory with the increase in the carriers injection level(rate) is limited mostly to the p-side (Red) and n-side (Blue) activelayers (QWs) with corresponding colors emission; respectively, andtotally missing the dominance of the Green color primary of the gamut atany injection level, thus unable to achieve full coverage of thestandard RGB color gamut. As can be seen from FIG. 5B, doping thebarriers between the active layers of the semiconductor light emittingdiode structure according to conventional approaches without IBLs causesthe light emission to be dominated by the Green and Blue emission activelayers (QWs) at the low and high injection levels, respectively, andcauses the dominance of the Red color primary of the gamut to be missingat any injection level. It is therefore not possible to achieve fullcoverage of the standard RGB color gamut. As can be seen from FIGS. 5Cand 5D, with IBLs incorporated into the active region of the LEDstructure, the color emission trajectory with the increase in theinjection level (rate) fully covers the standard RGB color gamut.

Selection of the composition of intermediate carrier blocking layers(IBLs) will now be described with reference to FIGS. 7A-7F. In thisregard, FIGS. 7A-7F illustrate the sensitivity of MQW injection to thedesign of the IBLs. FIGS. 7A-7F show the CIE chromaticity diagram of anRGB-IBL LED similar to that of RGB-IBL LED 300 in FIG. 3, except withdesign parameters of one of the IBLs modified. In FIGS. 7A-7F, similarto FIGS. 5A-5D, the circle markers indicate the starting point at thelowest injection, the square markers correspond to the highest injectionlevel, the triangles mark the injection level, and the crosses (“X”)indicate the primary colors of a standard RGB gamut (crosses 702, 706,710, 714, 718, 722 indicate red; crosses 701, 705, 709, 713, 717, 721indicate green; crosses 703, 707, 711, 715, 719, 723 indicate blue.)

FIGS. 7A-7C illustrate sensitivity of color control to the choice of anintermediate blocking layer placed between Green and Red active layers(QWs), herein referred to as IBL2, as affected by its materialcomposition. In FIGS. 7A-7C, an intermediate blocking layer placedbetween Blue and Green QWs herein referred to as IBL1, is absent and theband-gap of IBL2 located between Green and Red QWs is increased asaffected by increasing the Aluminum concentration within its compositionsystematically from that of FIG. 7A to that of FIG. 7C. FIGS. 7A-7C thusshow how the IBL2 band-gap as affected by its material composition isused to control Red-Green color balance.

FIGS. 7D-7F illustrate how the band-gap of the IBL1 as affected by itsmaterial composition is used to adjust the LED output Green-Blue colorbalance at a specific injection level. In FIGS. 7D-7F, IBL2 design isfixed (as in RGB-IBL LED 300 of FIG. 3) while Aluminum concentration inan undoped IBL1 is increased systematically from that of subplot D tothat of subplot F. FIGS. 7D-7F thus show how IBL1 band-gap as affectedby its material composition can be used to control Green-Blue colorbalance at a specified nominal injection of 10 A/cm² indicated bytriangle marker in subplots D-F. In an embodiment described below, thisfeature will be used to adjust the white-light LED emission to themaximum efficiency point.

Selection of the doping amount of an intermediate blocking layer (IBL1)placed between Blue and Green active layers will now be discussed withreference to FIGS. 8A-8D and FIGS. 9A-9D. In this regard, FIGS. 8A-8Dand FIGS. 9A-9D further illustrate the sensitivity of MQW injection tothe design of IBLs using the process of RGB-IBL LED structure design asan illustrative example. FIGS. 8A-8D illustrate CIE diagrams for anRGB-IBL LED with selected and fixed IBL1 and IBL2 compositions and withvarying level of IBL1 doping. FIGS. 9A-9D present the correspondinginjection dependencies of emission chromaticity coordinates. In thisregard, the embodiments of FIGS. 9A-9D correspond, respectively, to theembodiments of subplots FIGS. 8A-8D. As can be seen from FIGS. 8A-8D,either n or p doping of IBL1 would affect the color balance at theRed-Green or the Green-Blue sides, respectively. In the structure of anRGB-IBL designed according to the embodiment of FIG. 8C (also theembodiment in FIG. 5C), IBL1 is undoped in order to achieve improvedcoverage of a target color gamut represented by the cross marks in FIGS.8A-8D, where crosses 802, 806, 810, 814 indicate red, crosses 801, 805,809, 813 indicate green and crosses 806, 807, 811, 815 indicate Blue. InFIGS. 8A-8D, similar to the plots in FIGS. 5A-5D, the circle markersindicate the starting point at the lowest injection, the square markerscorrespond to the highest injection level, the triangles mark theinjection level. In FIG. 9, lines 903, 906, 909, 912 indicate red; lines902, 905, 908, 911 indicate green; and lines 901, 904, 907, 910 indicateblue.

Selection of the doping amount of an intermediate blocking layer (IBL2)placed between Green and Red active layers will now be discussed withreference to FIGS. 10A-10D. FIGS. 10A-10D further illustrate thesensitivity of MQW injection to the design of IBLs using the process ofRGB-IBL LED structure design as an illustrative example. In this regard,FIGS. 10A-10D present CIE diagrams for RGB-IBL LED with selected andfixed IBL1 and IBL2 compositions and with varying level of p-doping inIBL2. As can be seen from FIGS. 10A-10D, the most extended coverage ofthe desired color gamut through varying the carrier injection level(rate) is established when IBL2 p-doping is set at the level shown inFIG. 10C, below or above which the gamut coverage does not sufficientlyextend across the desired RGB standard color gamut. In the structurepresented in FIG. 4C, the doping level of IBL2 is Na=1.5×1018 cm⁻³ inorder to achieve the most extended coverage of a target color gamutrepresented by the Red, Green and Blue “X”-marks in FIGS. 10A-10D inwhich crosses 1002, 1006, 1010, 1014 indicate red; crosses 1001, 1005,1009, 1013 indicate green; and crosses 1003, 1007, 1011, 1015 indicateblue.

An embodiment will now be discussed with reference to FIG. 11A in whichadditional multi-color QWs are included in the active region. Asillustrated in FIG. 11A, the RGB gamut is controlled by includingadditional QWs and re-designing the intermediate carrier blocking layers(IBLs). In this embodiment, indicated by the RG(Aqua)B-IBL column inFIG. 11A (plots 1160 and 1165), instead of using one green-emitting QWwith nominal emission wavelength 525 nm, two QWs including agreen-emitting QW with nominal emission wavelength 531 nm and anaquamarine-emitting QW with nominal emission wavelength 512 nm are used,separated by an additional intermediate carrier blocking layer (IBL3).The RGB-IBL column of FIG. 11A (plots 1150 and 1155) corresponds to theembodiment of FIG. 3, in which the structure does not include anaquamarine-emitting QW.

Table 2A compares the designed IBLs in an LED structure without anaquamarine-emitting QW (RGB-IBL LED) and in an LED structure accordingto this embodiment including an aquamarine-emitting QW (RGAB-IBL LED).FIG. 11A also shows the CIE characteristics of each active QW indicatedby corresponding color. In this regard, lines 1118 and 1123, crosses1103 and 1111 and dots 1104 and 1112 indicate red; lines 1117 and 1122,arrow 1119, crosses 1102 and 1110, and areas 1101 and 1109 indicategreen; lines 1116 and 1121, arrow 1120, crosses 1105 and 1113, dots 1106and 1114, and area 1108 indicate blue. In FIG. 11A, the RGAB-IBLstructure is designed with strong blue-shift in aquamarine QW emissionto extend the green-emission range beyond the nominal injection level of10 A/cm².

TABLE 2A IBL-LED structures IBL IBL and IBL positions donors acceptorsRGB-IBL RGAB-IBL IBL (10¹⁷ (10¹⁸ LED LED composition cm⁻³) cm⁻³) IBL1Al_(0.05)Ga_(0.95)N 8.0 0.0 (Blue-Aqua) IBL1 Al_(0.07)Ga_(0.93)N 0.0 0.0(Blue-Green) IBL2 Al_(0.20)Ga_(0.80)N 0.0 1.0 (Aqua-Green) IBL2Al_(0.20)Ga_(0.80)N 0.0 1.5 (Green-Red) IBL3 Al_(0.02)Ga_(0.98)N 0.0 0.7(Green-Red)

Selection of carrier injection current will now be described withreference to FIGS. 11B-C. In the embodiment illustrated by FIGS. 11B-C,an additional intermediate carrier blocking layer (IBL0) in front(n-side) of the blue-emitting QW has been incorporated into an RGB-IBLLED structure disclosed herein, and the other intermediate blockinglayers (IBL1 and IBL2) and barrier layers of the structure's activeregion are re-designed. Accordingly, it is possible to improve the fullcolor current injection range of the RGB-IBL LED structure. FIG. 11Billustrates the band profile the RGB-IBL LED structure of thisembodiment. Similar to FIG. 4, the dashed lines in FIG. 11B show thequasi-Fermi levels for electrons (line 1124) and holes (line 1125). Thedashed-dotted line (line 1126) in FIG. 11B indicates the internalpotential distribution inside the active region.

Table 2B provides example parameters for design of the active regionaccording to the embodiment of FIGS. 11B-C. As shown in Table 2B,besides the addition of IBL0 at the n-side of the structure,re-designing the active region of the light emitting layout structureincludes: (i) adding IBL0 with an Aluminum content at 22% accompanied bylight Si doping at 7×1017 cm⁻³, (ii) increasing the barrier separationbetween B-QW and G-QW layers accompanied by Mg doping at 1×1017 cm⁻³,(iii) increasing the Aluminum content in IBL1 up to 23% accompanied bylight Mg doping at 0.3×1017 cm⁻³, and (iv) increasing the Aluminumcontent in IBL2 up to 25% accompanied by Mg doping at 5×1017 cm⁻³.

FIG. 11C shows the injection dependence of the light emission gamut CIERGB chromaticity coordinates of the RGB-IBL LED structure example ofthis embodiment. In FIG. 11C, triangle 1126 indicates green, square 1127indicates green and cross 1128 indicates green. Also, triangle 1129indicates red, square 1130 indicates red and the cross 1131 indicatesred. Square 1132 indicates blue, triangle 1133 indicates blue and cross1134 indicates blue. The dotted lines (line 1136 indicating Red; line1138 indicating Green; line 1140 indicating Blue) in FIG. 11C show theemission gamut of the RGB-IBL LED structure with the additional IBL0 butwithout re-designing of the remaining structure. The solid lines (line1135 indicating Red; line 1137 indicating Green; line 1139 indicatingBlue) in FIG. 11C show the emission gamut of the RGB-IBL LED structurewith the additional IBL0 and re-designing of the remaining structure. Ascan be seen from the solid lines FIG. 11C, the RGB-IBL LED structurewith the additional IBL0 and re-designed structure of this embodimentfully covers the standard RGB gamut within an injection current rangingfrom 20 mA/cm² to 50 A/cm², which is substantially narrower than theexamples of the previous embodiments.

An additional benefit of incorporating the intermediate carrier blockinglayers (IBLs) into the active region of a multi-color light-emittingdiode structure (such as active region 330 of the structure 300 in FIG.3) is that the IBLs also act as intermediate strain-compensation layers.As such, in addition to balancing the carrier transport across theactive region, incorporation of the IBLs also minimizes the crystallinestrain across a multiple band gap active region (such as active region330 of light-emitting diode structure 300). As a result, incorporationof the IBLs also promotes higher indium intake into nitride-based(InGaN) optically active layers (such as optically active layers 331 ofthe multi-color light-emitting diode structure 300), thus enabling theepitaxial growth of optically active layers having long wavelength lightemission ranging from Amber (615-nm) to Red (625-nm) within the activeregion of the multi-color light-emitting diode structure. Thus,incorporating the intermediate carrier blocking layers (IBLs) alsoenables production of a color-tunable nitride-based light-emitting diodestructure that has current injection controllable light emission acrossthe visible light spectrum.

TABLE 2B Donors Acceptors Width (10¹⁷ (10¹⁷ Layer nm Composition cm⁻³)cm⁻³) P-Layer 250 GaN 0.0 100.0 EBL 15.0 Al_(0.20)Ga_(0.80)N 0.0 500Barrier 10.0 GaN 0.0 5.0 Red QW 3.0 Ga_(0.65)In_(0.35)N 0.0 0.0 Barrier10 GaN 0.0 0.0 IBL2 (Green-Red) 10.0 Al_(0.25)Ga_(0.75)N 0.0 5.0 Barrier10 GaN 0.0 0.0 Green QW 3.0 Ga_(0.75)In_(0.25)N 0.0 0.0 Barrier 10.0 GaN0.0 0.0 IBL1 (Blue-Green) 10.0 Al_(0.23)Ga_(0.77)N 0.0 0.3 Barrier 25.0GaN 0.0 1.0 Blue QW 3.0 Ga_(0.85)In_(0.15)N 0.0 0.0 Barrier 10.0 GaN 0.00.0 IBL0 (Blue) 10 Al_(0.22)Ga_(0.78)N 7.0 0.0 Barrier 10.0 GaN 1.0 0.0N-Layer 500 GaN 10.0 0.0

FIG. 12A provides a comparison of the variable-color emission spectraobtained from simulation using an RGB-IBL LED (the RGB-IBL column ofFIG. 11A) with experimental electroluminescence spectra obtained fromthree-color Amber-Green-Blue-IBL LED epitaxially grown according to theRG(Aqua)B-IBL column of FIG. 11A. In the simulated spectra (plot 1200)the voltage drop across the active region changes from 3.0V to 3.7V with20 mV step. In plot 1200, moving from the line 1201 towards line 1202 onthe plot, line 1201 indicates purple, the next several lines indicateblue, then green, then yellow, then orange, then red, then repeats againstarting at purple until line 1202 indicates red. In the experimentaldata set (plot 1220) the legend indicates total LED injection current.In the legend of plot 1220, 4 mA indicates red, 7 mA indicates lightgreen, 11 mA indicates dark blue, 31 mA indicates light blue, 61 mAindicates purple, 101 mA indicates dark blue, 151 mA indicates lightblue, 201 mA indicates dark blue, 251 mA indicates light blue, 301 mAindicates red, 340 mA indicates dark green, 350 mA indicates lightgreen, and 353 mA indicates orange. Referring to plot 1220, line 1222corresponds to 4 mA indicating red, and line 1221 corresponds to 353 mAindicating orange. Moving from line 1222 up towards line 1221, theintermediate lines correspond, in order, to the legend of plot 1220.Accordingly, the line above line 1222 corresponds to 7 mA indicatinglight green, and the next line up corresponds to 11 mA indicating darkblue, and so on.

FIGS. 12B and 12C provide the variable-color emission spectra obtainedfrom an RGB-IBL LED designed in accordance with the disclosure hereinand epitaxially grown using III-nitride. In order to achieve the fullcolor gamut demonstrated in FIGS. 12B and 12C the epitaxially grownRGB-IBL LED structure includes the incorporation of an IBL in betweenQWs with the same emission wavelength. Specifically, the variable-coloremission spectra obtained from the epitaxially grown RGB-IBL LEDprovided in FIGS. 12B and 12C incorporated three Blue emission QWs andthe Blue-Green IBL1 then two Green emission QWs separated by anadditional IBL, designated as IBL1.5, followed by the Red emission QWwhich is separated from the second Green emission QW by IBL2. Thecomposition and doping of the three IBLs in this epitaxially grownRGB-IBL LED was selected as described above to achieve the color gamutcoverage at the injection range provided in FIGS. 12B and 12C. In FIG.12B, (a) at 5 mA demonstrates emission of red light, (b) at 20 mAdemonstrates emission of orange light, (c) at 30 mA demonstratesemission of yellow light, (d) at 100 mA demonstrates emission of lightgreen light, (e) at 200 mA demonstrates emission of light blue light,and (f) at 350 mA demonstrates emission of blue light. In FIG. 12C, 700nm and 640 nm indicate red; 620 nm indicates red-orange; 600 nmindicates orange; 590 nm indicates light orange; 580 nm indicates lightorange; 570 nm indicates yellow; 560 nm and 540 nm indicate light green;520 nm, 510 nm and 500 nm indicate green; 496 nm indicates blue; 480 nmindicates light blue; and 480 nm and 460 nm indicate dark blue.

FIG. 13 further specifies the experimental electroluminescence spectraof the monolithic color-tunable nitride-based three-color RGB-IBL LED300 of FIG. 3 designed and grown according to the disclosure herein andprovides a more detailed comparison with simulation results. Plots1321-1323 in FIG. 13 show the EL spectra measured at room temperature ata low, moderate, and high injection current density. For low currentdensity of about 0.5 A/cm² (plot 1323), the light emission is mainly inred color with wavelength ranging roughly from 560 to 650 nm and with afull width at half maximum (FWHM) around 50 nm. For moderate currentdensity of about 10 A/cm² (plot 1322), the light emission is mainly ingreen color with wavelength ranging roughly from 480 to 540 nm and witha full width at half maximum (FWHM) around 45 nm. For current density ofabout 50 A/cm² (plot 1321), the light emission is mainly in blue colorwith wavelength ranging roughly from 420 to 475 nm and with a full widthat half maximum (FWHM) around 35 nm. Plots 1334-1335 show correspondingresults for spectral emission power at three voltage bias values (inVolts). In plot 1334, line 1301 indicates a voltage bias value of 3.46Volts, line 1302 indicates a voltage bias value of 3.16 Volts, and line1303 indicates a voltage bias value of 2.98 Volts. Plot 1335 presentsthe results on a normalized scale. In plot 1305, line 1305 indicatesred, line 1306 indicates green and line 1304 indicates blue.

In the preceding embodiments multiplicity of intermediate carrierblocking layers (IBLs) are incorporated within the optically activeregion of the RGB-IBL LED. As described, in some of these embodimentsthe IBLs separate regions of active layers incorporating one or more QWshaving a specific wavelength emission. Also in some of these embodimentsan IBL separates active layers incorporating QWs having the samewavelength emission. Yet in other embodiments of this invention an IBLis incorporated at n-side of the optically active region of the RGB-IBLLED. In all of these embodiments the IBLs composition and doping areselected and designed as described in the preceding discussion to enablecontrollable (or tunable) color emission covering wide color gamut overa given carrier injection (I,V) range. The general design criteria ofthe IBLs composition and doping in these embodiments include one or moreof the following: (1) the conduction band (CB) energy level of the IBLsshould be higher than the CB edge of the optically active region andprogressively increasing from the n-side toward the p-side of thestructure; and (2) the valence band (VB) energy level of the IBLs shouldbe lower than the VB edge of the optically active region andprogressively decreasing from the n-side toward the p-side of thestructure.

According to the embodiments disclosed above, multi-color semiconductorlight emitting structures (for example, a color-tunable RGB-IBL LEDdevice) are designed by selection of one or more of the following designparameters of the semiconductor light-emitting structure: (1) theband-gaps of the active layers QWs being selected to achieve lightemission over a desired color emission gamut; (2) the number of activelayers QWs with a band-gap corresponding to the light emission at agiven wavelength being selected to include a desired color emissionprimaries into the device light emission gamut at a desired relativeoptical emission power for each color; (3) the multiplicity of IBLs withoptimized material composition and doping levels being incorporated intothe active region of the light emitting device in order to enablecarrier injection rate control of the device light emission color withinthe desired gamut; (4) the multiplicity of IBLs with optimized materialcomposition and doping levels being incorporated into the active regionof the light emitting device in order to enable control of the devicelight emission color within a given carrier injection range; and (5) theband-gap of the multiplicity of active layers QWs corresponding to thelight emission at a given wavelength being selected to provide a widegamut color emission.

Several examples will now be described with reference to FIGS. 14-16 inwhich the design parameters of the semiconductor light-emittingstructure are selected to provide for various applications. FIG. 14illustrates an embodiment in which the design parameters of the IBLsincorporated within a multi-color LED structure are selected to create amonolithic white LED with variable color temperature. FIG. 14 shows CIEchromaticity diagram 1400 and injection dependence of emissionchromaticity coordinates 1410. In the embodiment of FIG. 14, the designparameters of an RGB-IBL-White LED structure have been selected toproduce white-light emission at the maximum LED efficiency point at aninjection current density of 50 A/cm² (indicated by the invertedtriangle in 1400 of FIG. 14) at the white point (marked by the cross1303 in plot 1400 of FIG. 14) of the color gamut formed by emissioncolor primaries of the RGB-IBL structure active layers QWs. In plot 1400of FIG. 14, cross 1303 and dot 1304 indicate red; cross 1302 and area1301 indicate green; cross 1305 and dot 1306 indicate blue. In plot 1410of FIG. 14, line 1308 indicates red, line 1309 indicates green and line1307 indicates blue.

Table 3 compares the IBLs designed for the white-light emittingRGB-IBL-White LED with IBLs designed for the RGB-IBL LED 300 of FIG. 3(also presented in FIG. 4C and in FIG. 5C.)

TABLE 3 IBL-LED structures IBL IBL and IBL positions donors acceptorsRGB-IBL RGB-IBL- IBL (10¹⁷ (10¹⁸ LED White LED composition cm⁻³) cm⁻³)IBL Al_(0.24)Ga_(0.76)N 3.8 0.0 (Blue-Green) IBL Al_(0.07)Ga_(0.93)N 0.00.0 (Blue-Green) IBL Al_(0.06)Ga_(0.94)N 2.8 0.0 (Green-Red) IBLAl_(0.20)Ga_(0.80)N 0.0 1.5 (Green-Red)

Plot 1410 of FIG. 14 shows the injection dependence of emissionchromaticity coordinates of the white-light emitting RGB-IBL-White LED.In this embodiment, the design parameters of the RGB-IBL-White LEDstructure are selected to create white light emission of a given colortemperature, for example 6500° K, at the nominal injection currentdensity of 50 A/cm². As shown in plot 1410 of FIG. 14, when theinjection current is increased to a value higher than the nominalinjection current density of 50 A/cm², the emitted white light willinclude higher level of relative intensity contribution from the Blueactive layer QW, thus causing the emitted white light color temperatureto correspondingly increase to a value T₊ higher than the temperature ofthe white light emitted at the nominal injection current density of 50A/cm². Similarly, as shown in plot 1410 of FIG. 14, when the injectioncurrent is decreased to a value lower than the nominal injection currentdensity of 50 A/cm², the emitted white light will include higher levelof relative intensity contribution from the Green and Red active layersQWs, thus causing the emitted white light color temperature tocorrespondingly decrease to a value T. lower than the temperature of thewhite light emitted at the nominal injection current of 50 A/cm². TheRGB-IBL-white LED structure design parameters are selected to create aninjection current control range above and below the nominal injectioncurrent density that correspond to a desired white light emissiontemperature range around a nominal emitted white light temperature. Forexample, when a white light temperature of 6500° K is selected at thenominal injection current density of 50 A/cm², the RGB-IBL-white LEDstructure design parameters could also be designed to allow a range ofwhite color temperature ranging from 2500° K to 8000° K, for example,when the device injection current is decreased or increased over a givenrange below and above, respectively, the nominal injection currentdensity of 50 A/cm².

As can be seen from FIG. 14, design parameters are selected for IBLsthat are incorporated within the active region of a multi-color LEDstructure to create a monolithic white LED device whose emitted whitelight temperature is changed by changing its current injection level. Inthe general lighting field of application, the controllable (oradjustable) white emission temperature monolithic white LED device canbe used to create a solid state light bulb having a the controllable(adjustable) white emission temperature. In the display field ofapplication, the controllable (adjustable) white emission temperaturemonolithic white-light emitting LED device can be used to create a solidstate back light unit (BLU) for liquid crystal displays (LCD) or anyother types of displays that require back illumination light such asquantum dots displays. One advantage of using such a monolithic whiteLED as a back light for displays is that its brightness as well as itswhite color temperature can be adjusted to match the displayrequirements without sacrificing the dynamic range of the displayedimages. In that regard, in conventional white back lit displays, thebacklight color temperature, and often its intensity, are typicallyfixed and the displayed image brightness or hue are adjusted byadjusting the displayed image pixels RGB values; an approach thattypically results in a reduction of the displayed image dynamic rangesince a portion is being used to adjust the displayed image brightnessand hue rather than being used to express the grayscale value of eachpixel color. A display BLU that uses the controllable (adjustable) whiteemission temperature monolithic white-light emitting LED device can beused to alleviate these limitations and thus enable high dynamic range(HDR) displays.

FIG. 15 shows the electroluminescence spectra of the monolithicbroadband visible light-emitting nitride-based RGB-IBL LED 300 of FIG. 3epitaxially grown and designed according to the disclosure herein. Plot1500 of FIG. 15 illustrates an EL spectrum at a driving current densityof about 1 A/cm². The light emission is mainly in red band withwavelength range roughly from 560 to 680 nm and a full width at halfmaximum (FWHM) of 70 nm. Plot 1510 of FIG. 15 illustrates the ELspectrum at a medium driving current density of about 10 A/cm². Thelight emission is mainly in a combined red and green band withwavelength range roughly from 500 to 690 nm and a full width at halfmaximum (FWHM) of 120 nm. Plot 1520 of FIG. 15 illustrates the ELspectrum at injection current density of about 40 A/cm². The light isemitted in a combined red, green and blue bands with wavelength spanningroughly from 440 to 700 nm and a full width at half maximum (FWHM) of190 nm. As can be seen from FIG. 15, design parameters are selected forIBLs that are incorporated within the active region of a multi-color LEDstructure to create a monolithic white LED device having a broadbandwhite light emission.

FIG. 16 illustrates an embodiment in which the design parameters of theIBLs incorporated within a monochromatic LED structure are selected tocreate uniformly populated active layers within the LED structure, thusalleviating the uneven and imbalanced population of active QWs thatwould unfavorably affect the device performance (for example, in termsof efficiency). FIG. 16 compares monochromatic blue-emitting (plots 1620and 1640) and green-emitting (plots 1625 and 1645) MQW LEDs of theconventional structure of FIG. 1 (plots 1620 and 1625) with MQW-IBL LEDsdesigned according to the disclosure herein to sustain uniformlypopulated active layers in the MQW active region (plots 1640 and 1645).In each row, the subplots of FIG. 16 present distributions of opticalemission power among active QWs (P_(i)/Σ_(i)) in 3-QW LEDs according tothe conventional structure of FIG. 1 (plots 1620 and 1625) and 3-QWIBL-LEDs with IBLs selected for uniform emission distribution at LEDinjection level corresponding to maximum LED internal quantum efficiency(IQE) achieved at injection current density of 50 A/cm² (plots 1640 and1645). Conventional LED structures (such as FIG. 1) in plots 1620 and1625 include structures without EBL (solid line 1602 indicating blue,solid line 1604 indicating light blue, solid line 1606 indicatingpurple, solid line 1608 indicating green, solid line 1610 indicatinglight green, and solid line 1612 indicating dark green) and with p-sideEBL (dashed line 1601 indicating blue, dashed line 1603 indicating lightblue, dashed line 1605 indicating purple, dashed line 1609 indicatinggreen, dashed line 1611 indicating light green, dashed line 1613indicating dark green) thus illustrating the insignificant effect of theEBL on MQW emission uniformity except at the highest injection levelswhere the whole LED internal quantum efficiency (IQE) alreadydeteriorates. On the contrary, MQW-IBL LEDs reveal relatively uniformemission distribution in a wide range of practically important injectioncurrents (marked by the circles in plots 1640 and 1645) well exceedingthe initial optimization injection current of 50 A/cm². In plots 1640and 1645 of FIG. 16, line 1605 indicates blue, line 1606 indicates lightblue, line 1607 indicates purple, line 1614 indicates dark green, line1615 indicates light green and line 1616 indicates green.

Table 4 compares the emission uniformity characteristics of conventionalLEDs (without IBLs) and IBL-LEDs at nominal injection level of 50 A/cm²and shows IBL's Aluminum compositions and p-doping acceptorconcentrations selected according to this embodiment.

TABLE 4 IBL Aluminum IBL1/IBL2, IBL acceptors P_(min)/P_(max) LEDstructures % IBL1/IBL2, 10¹⁸ cm⁻³ at 50 A/cm² BBB (prior art) 0.15BBB-IBL 20/10 0.5/1.0 0.59 GGG (prior art) 0.06 GGG-IBL 20/15 1.2/1.50.63

As can be seen from FIG. 16, the LED structure can be designed to havean IQE at a desired current injection density within a low tointermediate range by selected design parameters for IBLs that areincorporated within the active region of a monochromatic LED structure.The embodiment of FIG. 16 is particularly advantageous in applicationsin which an array of micro-scale solid state light emitters is used asemissive pixels of a display, herein referred to as emissive micro-scalesolid state light display. In such an application, the entire emissiveaperture of the device is also its optical aperture, and the device cantherefore operate at a rather high optical coupling efficiency and thuswould not typically require the use of high current injection density.In contrast, typical solid state light applications generally requirethe use of rather high current injection density. Emissive micro-scalesolid state light displays would typically operate a low currentinjection regime in comparison with typical solid state lightapplications which generally operate a high current injection regime.For such low injection regime, the solid state micro-scale emitters IQEcan be designed at their typical current injection operating point inorder to preserve and further increase their operational efficiency. Inanother embodiment, the embodiment of FIG. 16 is used to design the IQEof emissive micro-scale solid state light displays in order to increasetheir operational efficiency.

According to the methods and structures described herein, andparticularly by incorporating IBLs within the active region of amulti-color solid state light emitting structure, it is possible toprovide a variable-color emission (or tunable) solid state lightemitting material and variable-color emission (or tunable) solid statelight emitting devices. As discussed above, there are numerous fields ofapplication for such variable-color emission (or tunable) solid statelight emitting materials and devices, including general lighting anddisplay applications. As one example, the variable-color emission (ortunable) solid state light emitting materials and devices disclosedherein is in the field of emissive micro-scale solid state light displaydescribed in U.S. Pat. Nos. 7,623,560, 7,767,479, 7,829,902, 8,049,231and U.S. Patent Publication Nos. 2010/0066921 and 2012/0033113, thecontents of each of which are incorporated herein by reference. In thistype of display, micro-scale pixel multi-color emission is realized bystacking multiple layers of state light emitting structures on top of acontrolling silicon based complementary metal oxide semiconductor (CMOS)structure to form an individually addressable (in color and intensity)emissive micro-pixel array device. One advantage of such emissivemicro-scale solid state light display technology is that it can be usedto realize small pixel size in the range of few microns. How small thepixel pitch of such an emissive micro-scale solid state light displaycan be made is dependent upon the number electrical contacts needed tocontrol emission color and intensity of each micro-scale pixel. In thecase when three color primaries are used to create the display gamut,beside the one common contact for the entire micro-pixel array, at leastthree contacts are required per each emissive micro-scale pixel, whichbased on current semiconductor equipment capabilities allows therealization of a micro-scale pixel pitch in the range of approximately10 microns. When the variable-color emission (or tunable) solid statelight emitting material disclosed herein is used in the context of thistype of emissive micro-scale solid state light display, beside the onecommon contact for the entire micro-pixel array, only one contact permicro-scale pixel is needed to control each pixel light emission. Thisreduction in the number of required contacts per micro-pixel makes itadvantageously possible to realize an emissive multi-color micro-scalepixel pitch of 5 microns or less. In addition, when the variable-coloremission (or tunable) solid state light emitting material disclosedherein is used in the context of this type of emissive micro-scale solidstate light display, only one solid state light emitting layer is neededto realize a multi-color emissive micro-scale pixel display (instead ofthe typical three layers), which in turn will substantially reduce themanufacturing cost of the display.

While the invention has been described in terms of several embodiments,those of ordinary skill in the art will recognize that the invention isnot limited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. The description is thus to be regarded as illustrative insteadof limiting. There are numerous other variations to different aspects ofthe invention described above, which in the interest of conciseness havenot been provided in detail. Accordingly, other embodiments are withinthe scope of the disclosure herein, aspects of which are defined in theclaims.

What is claimed is:
 1. A multi-layer semiconductor light emittingstructure comprising: a first quantum confinement structure for emittinglight of a first wavelength; a first intermediate carrier blockinglayer; a second quantum confinement structure for emitting light of asecond wavelength, wherein the first intermediate carrier blocking layeris disposed between the first quantum confinement structure and thesecond quantum confinement structure and the first intermediate carrierblocking layer comprises a composition that is undoped or doped of asingle conductivity type to provide a first predetermined band-gap andband-offsets relative to each of the first quantum confinement structureand the second quantum confinement structure; a second intermediatecarrier blocking layer ; a third quantum confinement structure foremitting light of a third wavelength, wherein the second intermediatecarrier blocking layer is disposed between the second quantumconfinement structure and the third quantum confinement structure andthe second intermediate carrier blocking layer comprises a compositionthat is undoped or doped of a single conductivity type to provide asecond predetermined band-gap and band-offsets relative to each of thesecond quantum confinement structure and the third quantum confinementstructure; and the first, second and third quantum confinementstructures emitting primary colors of red, green and blue respectively,wherein the band-gaps and the band-offsets of the first and secondintermediate carrier blocking layers are selected to cause the first,second and third quantum confinement structures to emit light of therespective wavelength and with a relative intensity contribution tocause the multi-layer semiconductor light emitting structure to emitwhite light.
 2. The multi-layer semiconductor light emitting structureof claim 1, wherein only one of the composition of the firstintermediate carrier blocking layer and the composition of the secondintermediate carrier blocking layer is doped and the other is undoped.3. The multi-layer semiconductor light emitting structure of claim 1wherein at least one of the first and second quantum confinementstructures comprises a quantum wire.
 4. The multi-layer semiconductorlight emitting structure of claim 1 wherein at least one of the firstand second quantum confinement structures comprises a quantum dot. 5.The multi-layer semiconductor light emitting structure of claim 1wherein at least one of the first and second quantum confinementstructures comprises a quantum well.
 6. The multi-layer semiconductorlight emitting structure of claim 1 wherein at least one of the first,second and third quantum confinement structures comprises multiplequantum wells.
 7. The multi-layer semiconductor light emitting structureof claim 1 wherein the first, second and third quantum confinementstructures emit the respective colors with approximately equal CIE at aninjection current density of approximately 50 A/cm².
 8. The multi-layersemiconductor light emitting structure of claim 1 wherein the first,second and third quantum confinement structures emit red, green and bluelight with approximately equal CIE emissions for a particular injectioncurrent density, and when the particular injection current densityincreases, the blue emission increases and the green emission decreases.9. The multi-layer semiconductor light emitting structure of claim 8wherein an emission color temperature of the white-light-emitting deviceis controllable by controlling the injection current density.
 10. Themulti-layer semiconductor light emitting structure of claim 8 whereinthe first, second and third quantum confinement structures emit therespective colors with approximately equal CIE emissions when the redemission is also decreasing with increasing injection current density.11. The multi-layer semiconductor light emitting structure of claim 10wherein the emission color temperature of the multi-layer semiconductorlight emitting structure emitting white light is controllable bycontrolling the injection current density.
 12. The multi-layersemiconductor light emitting structure of claim 1, wherein the first,second and third quantum confinement structures are disposed between ann-doped layer and a p-doped layer and the multi-layer semiconductorlight emitting structure further comprises a first contact coupled tothe n-doped layer and a second contact coupled to the p-doped layer, andwherein a carrier injection into each of the first, second and thirdquantum confinement structures is achieved through the first contact andthe second contact.
 13. A multi-layer semiconductor light emittingstructure comprising: a first quantum confinement structure for emittinglight of a first wavelength, the first quantum confinement structurecomprising at least one first optically active layer disposed betweenbarrier layers; a first intermediate carrier blocking layer; a secondquantum confinement structure for emitting light of a second wavelength,the second quantum confinement structure comprising at least one secondoptically active layer disposed between barrier layers; a secondintermediate carrier blocking layer; and a third quantum confinementstructure for emitting light of a third wavelength, the third quantumconfinement structure comprising at least one third optically activelayer disposed between barrier layers, wherein the first intermediatecarrier blocking layer is disposed between the first quantum confinementstructure and the second quantum confinement structure and adjacent to abarrier layer of the first quantum confinement structure and adjacent toa barrier layer of the second quantum confinement structure and whereinthe first intermediate carrier blocking layer comprises (1) a firstband-gap greater than a band-gap of the adjacent barrier layer of thefirst quantum confinement structure and greater than a band-gap of theadjacent barrier layer of the second quantum confinement structure and(2) band-offsets relative to energy bands of the adjacent barrier layerof the first quantum confinement structure and relative to energy bandsof the adjacent barrier layer of the second quantum confinementstructure, wherein the second intermediate carrier blocking layer isdisposed between a barrier layer of the second quantum confinementstructure and a barrier layer of the third quantum confinement structureand adjacent to a barrier layer of the second quantum confinementstructure and adjacent to a barrier layer of the third quantumconfinement structure and wherein the second intermediate carrierblocking layer comprises (1) a second band-gap greater than a band-gapof the adjacent barrier layer of the second quantum confinementstructure and greater than a band-gap of the adjacent barrier layer ofthe third quantum confinement structure and (2) band-offsets relative toenergy bands of the adjacent barrier layer of the second quantumconfinement structure and relative to energy bands of the adjacentbarrier layer of the third quantum confinement structure, wherein thefirst, second and third quantum confinement structures emitting primarycolors of red, green and blue, respectively, and wherein the first andsecond intermediate carrier blocking layers each comprise a compositionthat is undoped or doped of a single conductivity type to provide thefirst and second band-gaps and band-offsets selected to cause the first,second and third quantum confinement structures to emit light of theprimary colors with a relative intensity to form white light.
 14. Themulti-layer semiconductor light emitting structure of claim 13 whereinat least one of the first and second quantum confinement structurescomprises a quantum wire.
 15. The multi-layer semiconductor lightemitting structure of claim 13 wherein at least one of the first andsecond quantum confinement structures comprises a quantum dot.
 16. Themulti-layer semiconductor light emitting structure of claim 13 whereinat least one of the first and second quantum confinement structurescomprises a quantum well.
 17. The multi-layer semiconductor lightemitting structure of claim 13 wherein at least one of the first, secondand third quantum confinement structures comprises multiple quantumwells.
 18. The multi-layer semiconductor light emitting structure ofclaim 13 wherein the first, second and third quantum confinementstructures emit the respective colors with approximately equal CIE at aninjection current density of approximately 50 A/cm².
 19. The multi-layersemiconductor light emitting structure of claim 13 wherein the first,second and third quantum confinement structures emit red, green and bluelight with approximately equal CIE emissions for a particular injectioncurrent density, and when the particular injection current densityincreases, the blue emission increases and the green emission decreases.20. The multi-layer semiconductor light emitting structure of claim 19wherein an emission color temperature of the white-light-emitting deviceis controllable by controlling the injection current density.
 21. Themulti-layer semiconductor light emitting structure of claim 19 whereinthe first, second and third quantum confinement structures emit therespective colors with approximately equal CIE emissions when the redemission is also decreasing with increasing injection current density.22. The multi-layer semiconductor light emitting structure of claim 21wherein the emission color temperature of the multi-layer semiconductorlight emitting structure emitting white light is controllable bycontrolling the injection current density.
 23. The multi-layersemiconductor light emitting structure of claim 13, wherein the first,second and third quantum confinement structures are disposed between ann-doped layer and a p-doped layer and the multi-layer semiconductorlight emitting structure further comprises a first contact coupled tothe n-doped layer and a second contact coupled to the p-doped layer, andwherein a carrier injection into each of the first, second and thirdquantum confinement structures is achieved through the first contact andthe second contact.
 24. The multi-layer semiconductor light emittingstructure of claim 13, wherein only one of the composition of the firstintermediate carrier blocking layer and the composition of the secondintermediate carrier blocking layer is doped and the other is undoped.